Three-phase slurry reactors are well known to those skilled in the art. In operation, the said reactors typically comprise a slurry zone and a freeboard zone. In the slurry, present in the slurry zone, the solid catalyst particles are kept in suspension in the liquid. The liquid serves amongst others as a heat-transfer medium. One or more gaseous reactants bubble through the slurry. The freeboard zone, usually located above the slurry zone, contains primarily gaseous products and/or reactants and substantially no slurry.
The catalyst particles are typically kept in suspension by stirring or agitation by a mechanical device or, preferably, by an upward gas and/or liquid velocity.
The mixture of hydrogen and carbon monoxide is commonly referred to as synthesis gas. The preparation of heavy hydrocarbons from synthesis gas is commonly referred to as Fischer-Tropsch synthesis. The term heavy hydrocarbons as used herein refers to hydrocarbons which are in the liquid state under reaction conditions. In this respect, it will be appreciated by that the Fischer-Tropsch synthesis not only yields heavy hydrocarbons, but also hydrocarbons which are gaseous under reaction conditions and oxygenates.
In particular, the present invention relates to the separation from the slurry of heavy hydrocarbons produced by the Fischer-Tropsch synthesis.
A number of ways have been proposed to separate liquid, in particular heavy hydrocarbons, from the slurry. Thus, European patent application publication No. 0 609 079 describes a slurry bubble column containing a slurry bed of catalyst particles suspended in a liquid. A filtration zone is located in the slurry bed, in particular close to the upper surface of the slurry bed. The filtration zone typically comprises a plurality of filter elements. The filter elements are typically of elongate cylindrical form and comprise a cylindrical filtering medium enclosing a filtrate collection zone.
European patent application publication No. 592 176 describes a filtration zone consisting of a tube sheet holding filter cartridges. The tube sheet defines the upper surface of the slurry bed.
International (PCT) application publication No. 94/16807 describes a filtration zone surrounding the slurry bed.
UK patent application publication No. 2 281 224 discloses a reactor containing a plurality of reaction tubes arranged to accommodate the slurry bed. The upper part of each tube contains a filter element to separate hydrocarbon product from slurry, and a top part of increased diameter, often referred to as a disengagement zone, to separate gas from the slurry.
U.S. Pat. No. 4,605,678 describes separation of catalyst from a slurry containing hydrocarbon product by passing the slurry through a high gradient magnetic field.
U.S. Pat. No. 5,324,335 describes a process for the preparation of hydrocarbons, using an iron-based catalyst. To avoid the continuous increase of slurry height in the reactor vessel, due to the production of heavy hydrocarbon waxes, wax is separated from the slurry using a cross-flow filter located outside the reactor vessel.
German patent No. 32 45 318 (DE-32 45 318) describes a process for separating a liquid product stream from a slurry, by cross-flow filtration, which is carried out at substantially reactor pressure, but outside the reactor. According to one embodiment, the slurry is cooled prior to filtration.
It has now been found that cross-flow filtration is considerably facilitated if the cross-flow filtration step is preceded by a degasification step, which step is preferably carried out in a device in which separation takes place in a centrifugal field, more preferably a hydrocyclone. Furthermore, in this way the control of the flow of filtrate through the cross-flow filter is considerably facilitated. In addition, it will be appreciated that the required filter area is less.